The present disclosure relates to optical measurement systems and methods of use. More particularly, the present invention is directed to an optical system for mitigating drift, fade and errors due to due to absorption and solarization effects in a flashlamp driven optical measurement system and detecting and correcting any remaining error.
Optical measurement systems are employed in a variety of industries, such as the semiconductor processing industry, for real-time monitoring of workpiece modification and process control. Optical measurement systems may be integrated with a semiconductor processing tool and utilized in-situ for real-time process control or may be used in-line for feedback control. Typically, monitored processes include semiconductor etching, deposition and CMP processes for film thickness and plasma monitoring applications.
Especially in the semiconductor processing industry, the use of increasingly thinner material layers and smaller features sizes, in accordance with Moore's Law, has led to a need to interrogate these small features with increasingly shorter wavelengths and tighter repeatability to achieve desired levels of measurement accuracy and precision. Shorter wavelengths of light in the UV and DUV regions of the spectrum (i.e., wavelengths less than 400 nm) cannot consistently be transmitted in optical fibers due to absorption and solarization effects. To utilize these short wavelengths for measurement, the use of optical fibers must be minimized or optical measurement systems must be alternatively free-space coupled. However, the removal of optical fibers or use of free-space coupling imposes considerable design challenges and limits the integrability of measurement systems.
FIG. 1 shows a pictorial schematic of a prior art optical measurement system 100. Optical measurement system 100 includes light analyzing device 110, light source 120, optical assembly 130, optical fiber assembly 140, computer 150 and workpiece 160. Light analyzing device 110 is commonly a spectrograph, spectrometer, monochromator or other light analyzing device providing wavelength discrimination. Light source 120 is either a continuous broadband emission source (e.g., tungsten halogen lamp or deuterium lamp) or a pulsed broadband emission source such as a xenon flashlamp. Optionally, narrowband continuous or pulsed emission sources such as lasers are used. Optical assembly 130 is designed to direct light of one or more wavelengths emitted from light source 120 onto workpiece 160 which is a silicon semiconductor wafer, sapphire substrate or other workpiece. Optical assembly 130 commonly acts to either focus or collimate light from light source 120 onto workpiece 160. Optical fiber assembly 140 is commonly a bifurcated optical fiber assembly which directs light from light source 120 to workpiece 160 via optical assembly 130 and subsequently directs light collected upon reflection from workpiece 160 via optical assembly 130 to light analyzing device 110. Computer 150 is used to control light analyzing device 110 and light source 120 and is also used to analyze data collected by light analyzing device 110. Computer 150 may also provide signals to control external systems such as processing tools (not shown).
UV and DUV radiation adversely affects light analyzing device 110, light source 120, optical assembly 130 and optical fiber assembly 140 such that analysis performed by computer 150 of reflected light signals from workpiece 160 will contain errors (e.g., drifts, variations, noise, signal fade) and result in misprocessing of workpieces. UV and DUV radiation exposure causes measurement errors as a result of solarization of optical fibers, degradation of measurement system elements, and absorption by oxygen and the creation of and absorption by ozone in the optical signal path. In addition to the effects of UV and DUV, optical measurement system 100 is subject to other sources of variation that contribute to measurement error and/or drift including detector variation, CCD aging, lamp variation, lamp aging and mechanical dis-alignment or variation.